The demand for high-density, cost-effective printed circuit boards has prompted the electronics industry to seek alternative methods to traditional plated-through-hole technologies. One such alternative is surface mounting, a technology in which the components have leads that allow soldering to a surface of the printed circuit board, without the necessity of using plated-through holes. Surface mounting is a cost effective technology because it offers higher density component placement on the printed circuit board.
One such surface mounted component is illustrated in FIGS. 1 and 2. FIG. 1 is a top view of a flatpack component 10, including a body 12 and leads 14. FIG. 2 shows the flatpack 10 in a side elevation after the leads 14 have been shaped to form a foot 16 in preparation for soldering to a printed circuit board.
In preparing the flatpack leads it is critical that the foot 16 of each lead 14 be in the same plane so that when the flatpack 10 is placed on the printed circuit board each lead contacts a solder pad for later soldering. It is also critical for each of the leads to be properly positioned in the X-Y direction, i.e., the distance from the flatpack body to the foot 16 and the distance between the leads 14. Again, this ensures that each lead 14 contacts the proper pad on the printed circuit board. If the leads 14 are not properly positioned when the flatpack 10 is applied to the printed circuit board, the soldering operation may produce cold solder joints or bridges between the leads 14 and/or the pads on the printed circuit board. Also, an improperly positioned flatpack could subJect the solder joints to undue stresses that may eventually cause the joint to crack, creating an open circuit. For systems designed to MIL-SPEC DOD 2000 standards, there is also a tolerance requirement for placement of the foot 16 on the pad relative to the dimensions of the pad.
FIGS. 3 through 6 illustrate the prior art technique for forming and cutting the flatpack leads 14. FIG. 3 shows the body 12 and the leads 14 extending horizontally therefrom; this is the configuration in which the flatpack is received from the fabrication process. The lead 14 is held between a clamp 20 and a forming anvil 22. A blade 24, driven by a cylinder not shown in FIG. 3, moves downwardly against the lead 14 to form the lead 14, as shown in FIGS. 4 and 5. In FIG. 4 the downward progress of the lead 14 stops when the lead 14 strikes a corner 25 of the forming anvil 22. In fact, the lead 14 is scratched when it strikes the corner 25 resulting in some metal removal. At this point, as the blade 24 continues to move downwardly the lead 14 conforms to the shape of the corner 26 within the forming anvil 22. This process of forming the lead 14 against the forming anvil 22 produces coining in the lead 14. Coining is a stretching or a reduction in lead thickness caused when the blade 24 stretches the lead 14 as it is made to conform to the shape of the corner 26. As is known by those skilled in the art, this coining produces unnecessary residual stresses in the lead 14 that may cause the lead 14 to break while it is being soldered in place or, even worse, during its service life. When the flatpack is soldered in place, these residual stresses in the lead also place undue stresses on the solder joint itself. Thus, one of the objects of the present invention is to overcome this disadvantage by providing a two-stage forming process that does not produce coining of the leads 14.
After the lead 14 has been properly shaped a cutting blade 28 moves downwardly (See FIG. 6) to cut the excess material from the lead 14. This prior art process may result in downward flagging of the lead 14, where a small portion of the lead 14 is bent downwardly during the cutting operation. Flagging is especially prevalent when the cutting blade 28 is dull, as dull blade drags the metal down instead of making a clean cut. The flagged portion is identified by reference character 30 in FIG. 6A, and can be seen in the close-up view of FIG. 6B. When a flatpack 10 cut in accord with the prior art is laid upon a printed circuit board for soldering, the flagged portion 30 prevents the entire foot 16 from contacting the printed circuit board trace, thus resulting in a weak solder joint. Because this prior art forming and cutting process requires only one step (i.e., the blade 24 and cutting blade 28 can be driven downwardly simultaneously in a single step), it is simple and thus inexpensive to implement. The present invention, although more complex, produces a vastly improved final product.